:find conditions using hour value of saved DateTime object

rubyonrails-talk
1

I am wondering if it is possible to use a find condition to find objects according to the hour, ignoring the date. For example, i have a date field storing DateTime objects in the database and I want to find all objects where the hour is between the times 2:00PM and 4:00PM regardless of their date. It seems that to do this, ActiveRecord would have to perform an operation on each DateTime to find the hour, such as:

find(:all, :conditions => ["date.hour >= ? AND date.hour <= ?", 2:00PM, 4:00PM])

I know this does not work, but it helps to convey my goal. Maybe I have to write a raw SQL condition? Maybe it is easier to just save the time values in a separate field for each object in the database in the firstplace? I know saves are expensive and was trying to avoid them.

2

Ideally, you should save this in separate columns, as this query is definitely going to kill your database (there's no way to make an index for it, the database will have to go through all rows), but you could do it like this if you're using MySQL:

find(:all, :conditions => ["HOUR( date ) >= ? AND HOUR( date ) <= ?", 2, 4])